Medical physics (A-level only)

State the type of retinal receptor that is mainly responsible for vision at very low light intensity and give one property of the resulting image.

Explain why two faint points of light that fall on nearby rod cells may be seen as one point even though both points are detected.

A coloured grid is viewed first in bright light and then after the illumination is greatly reduced. Explain the changes expected in colour, detail and sensitivity, referring to the retinal receptors and their nerve connections.

State the type of spectacle lens used to correct myopia and state the sign of its power.

A converging lens has power $+5.0\,\text{D}$. An object is $0.60\,\text{m}$ from the lens. Using the real-is-positive convention, calculate the image distance and the magnitude of the magnification.

A hypermetropic eye has a near point $0.80\,\text{m}$ from a spectacle lens. Determine the lens power needed so that an object $0.25\,\text{m}$ from the lens forms a virtual image at the near point. Use the real-is-positive convention.

Name the membrane that vibrates when sound reaches the end of the ear canal and name the three-bone system moved by it.

Explain two features of the middle ear that increase the pressure delivered to the fluid in the cochlea.

A sound produces pressure amplitude $0.012\,\text{Pa}$ on a tympanic membrane of area $5.5\times10^{-5}\,\text{m}^2$. The ossicles increase the force by a factor of $1.4$ and act on an oval window of area $3.2\times10^{-6}\,\text{m}^2$. Calculate the pressure amplitude delivered to the inner-ear fluid.

Calculate the intensity level of a sound with intensity $1.0\times10^{-8}\,\text{W m}^{-2}$. Use $I_0=1.0\times10^{-12}\,\text{W m}^{-2}$.

Two sounds have intensity levels $76\,\text{dB}$ and $58\,\text{dB}$. Determine the ratio of the larger intensity to the smaller intensity.

An isotropic sound source has power $0.80\,\text{W}$. Determine the distance at which its intensity level is $85\,\text{dB}$. Then explain why a $12\,\text{kHz}$ tone at this level may sound quieter than a $3\,\text{kHz}$ tone at the same level. Use $I_0=1.0\times10^{-12}\,\text{W m}^{-2}$.

State the frequency range in which age-related hearing deterioration is usually greatest.

At $4.0\,\text{kHz}$, a normal threshold is $8\,\text{dB}$ and an impaired threshold is $38\,\text{dB}$. Calculate the hearing loss and the factor by which the threshold intensity has increased.

Listener A has hearing losses of $6\,\text{dB}$ at $0.5\,\text{kHz}$, $15\,\text{dB}$ at $4\,\text{kHz}$ and $44\,\text{dB}$ at $10\,\text{kHz}$. Listener B has losses of $9\,\text{dB}$, $48\,\text{dB}$ and $17\,\text{dB}$ at the same frequencies. Deduce the likely cause for each pattern and explain how each pattern changes an equal-loudness curve and perceived sound.

Identify the electrical events represented by the QRS complex and the T wave in a normal ECG.

Five successive R peaks occur at $0.42\,\text{s}$, $1.20\,\text{s}$, $1.98\,\text{s}$, $2.76\,\text{s}$ and $3.54\,\text{s}$. Determine the mean heart rate in beats per minute.

An ECG trace contains slow baseline changes when the patient moves and a regular interference signal from nearby mains equipment. Explain how electrode attachment, the leads and the amplifier should be arranged to obtain a clearer normal waveform.

A display shows echo amplitude against time rather than a two-dimensional image. Identify the ultrasound scan type and state what a B-scan displays instead.

Ultrasound passes normally from tissue A, where $\rho=1000\,\text{kg m}^{-3}$ and $c=1500\,\text{m s}^{-1}$, into tissue B, where $\rho=1060\,\text{kg m}^{-3}$ and $c=1540\,\text{m s}^{-1}$. Calculate the percentage of incident intensity reflected at the boundary.

A $4.0\,\text{MHz}$ ultrasound pulse travels through tissue at $1540\,\text{m s}^{-1}$ and returns to the transducer $130\,\mu\text{s}$ after transmission. Calculate the reflector depth and estimate the scan resolution as one wavelength. Explain why coupling gel is placed between the transducer and skin.

Name the endoscope fibre bundle that carries an image and the bundle that carries illumination into the body.

A fibre has core refractive index $1.62$ and cladding refractive index $1.50$. Calculate the critical angle at the core-cladding boundary and determine whether a ray incident there at $72.0^\circ$ undergoes total internal reflection.

A damaged endoscope has an image bundle whose fibre positions are rearranged between its two ends, and some cladding has been removed. Explain the effects on the observed image and why the illumination bundle can still work when its fibre order is random.

State which nuclei provide the main signal in an MR scanner and describe their behaviour in the scanner's static magnetic field.

Describe what happens to selected protons during and immediately after a short radio-frequency pulse in an MR scan.

Outline how an MR scanner obtains a cross-sectional image, beginning with a patient in the main magnetic field and ending with a computer-generated image.

State the maximum photon energy, in $\text{keV}$, from an X-ray tube operating at $68\,\text{kV}$.

An X-ray tube uses an accelerating potential difference of $72\,\text{kV}$. Calculate the minimum X-ray wavelength. Use $e=1.60\times10^{-19}\,\text{C}$, $h=6.63\times10^{-34}\,\text{J s}$ and $c=3.00\times10^8\,\text{m s}^{-1}$.

A rotating-anode tube operates at $95\,\text{kV}$ with a current of $3.0\,\text{mA}$ for $0.080\,\text{s}$. It converts $1.2\%$ of the electrical energy into X-rays. Calculate the maximum photon energy and the total X-ray energy produced. Explain one benefit of rotating the anode. Use $e=1.60\times10^{-19}\,\text{C}$.

State the function of the scintillator in a flat-panel X-ray detector.

Explain why a patient is given a barium suspension before an X-ray image of the digestive tract is recorded.

A clinic needs a single abdominal X-ray image that can be enhanced immediately while keeping patient dose as low as practicable. Discuss the suitability of photographic film, fluoroscopic image intensification and a flat-panel detector, and justify a choice.

An absorber has linear attenuation coefficient $0.28\,\text{cm}^{-1}$. Calculate its half-value thickness.

A narrow X-ray beam of intensity $6.4\,\text{W m}^{-2}$ passes through $7.0\,\text{mm}$ of tissue with $\mu=0.18\,\text{mm}^{-1}$. Calculate the transmitted intensity.

An incident X-ray beam has intensity $10.0\,\text{W m}^{-2}$. One ray crosses $18\,\text{mm}$ of soft tissue with $\mu=0.050\,\text{mm}^{-1}$ and then $4.0\,\text{mm}$ of bone with $\mu=0.32\,\text{mm}^{-1}$. A neighbouring ray crosses $22\,\text{mm}$ of the same soft tissue only. Calculate both transmitted intensities and the ratio of the soft-tissue-only intensity to the bone-path intensity.

State why the X-ray tube moves around a patient during a CT scan.

Describe how a CT scanner produces a cross-sectional image from X-rays transmitted through a patient.

A hospital is choosing between CT and a plain X-ray image to localise a small lung lesion partly hidden by overlapping ribs, and to confirm the position of a radiopaque feeding tube. Compare the techniques using image resolution, superposition, radiation dose, cost and availability. Recommend one technique for each task.

State why a medical tracer used for external imaging should emit gamma radiation.

Justify the use of technetium-99m for imaging an organ. Refer to its $6\,\text{h}$ half-life, its $140\,\text{keV}$ gamma emission and its chemical use.

Explain how a PET scan can map regions of high glucose uptake after a patient receives a positron-emitting glucose analogue. Include the nuclear event, the detection method and how position is inferred.

Define the biological half-life of a tracer in an organ.

A tracer has physical half-life $18\,\text{h}$ and biological half-life $30\,\text{h}$. Calculate its effective half-life.

A radionuclide has physical half-life $12.0\,\text{h}$. Measurements in a patient give an effective half-life of $7.20\,\text{h}$. Calculate the biological half-life and the percentage of the initial activity remaining in the patient after $21.6\,\text{h}$.

State the function of the lead collimator in a gamma camera.

Describe how a gamma photon entering a gamma camera produces an electrical pulse at the output of a photomultiplier tube.

Explain how a gamma camera determines the position of tracer activity and improves the quality of the recorded image. Include the roles of the collimator, photomultiplier array and pulse-height selection.

State why high-energy X-rays are used to treat a tumour deep inside the body.

Explain how directing X-ray beams at a tumour from several angles can reduce damage to healthy tissue.

A tumour of mass $0.18\,\text{kg}$ receives $1.8\,\text{Gy}$ in each of $20$ X-ray treatment fractions. Calculate the total energy absorbed by the tumour. Explain two methods, other than reducing the prescribed tumour dose, that limit exposure of healthy cells.

State why a beta emitter is suitable for a radioactive implant placed inside a tumour.

Explain why a beta-emitting implant may expose less healthy tissue than an external high-energy X-ray beam used to deliver the same tumour dose.

A temporary beta-emitting implant has initial activity $9.6\,\text{MBq}$ and physical half-life $48\,\text{h}$. Calculate its activity after $120\,\text{h}$. Explain how the position and radiation type of the implant help protect healthy tissue.

Select a suitable imaging technique for repeated bedside monitoring of moving soft tissue when ionising radiation must be avoided.

Compare CT and MR for repeated imaging of a brain tumour. Refer to soft-tissue contrast, patient dose and convenience.

A patient may have a kidney stone. Evaluate ultrasound, CT and MR for locating the stone, using spatial resolution, soft-tissue or stone contrast, radiation dose, availability and convenience. Reach a justified recommendation.